Metallic coatings may be applied to substrates using a thermal-spray process. In this process, a coating material, usually provided in a power or wire form, is heated to an elevated temperature in a spray device. The coating material may be entirely melted to form liquid droplets, may be partially melted to form semiplastic particles, or may be unmelted solid powder particles. The heated coating material is ejected from the spray device at a high velocity and thence sprayed against a substrate article surface. The sprayed material deposits upon the surface and, to the extent that it is liquid, solidifies. The droplets and particles impact the surface at a high velocity, and are flattened against the surface. The deposition continues until the solidified coating has reached a desired thickness, often as great as about 0.150 inches.
The thermal-spray process is highly versatile. It may be used with a wide variety of compositions and substrate articles, yielding a variety of different types of properties. In one example, to build up an article that has been partially worn away during prior service, the coating material may have the same compositions as the substrate article. In another example, to provide a wear-resistant coating at the surface, the coating has a different composition than the substrate article and is more wear resistant than the substrate article. In yet another example, to provide a wearing or abradable coating at the surface, the coating has a different composition than the substrate article and is less wear resistant than the substrate article. The thermal-spray process may be used to coat irregular and complexly shaped article substrates.
For most of its applications, the thermally sprayed coating must adhere very well at a bondline to the entire surface to which it is applied, with a good mechanical bond. There may not be delaminations of the coating from the substrate, where a region of the coating is completely separated from the substrate. In some more-demanding applications, the coating must further be metallurgically bonded to the substrate.
The usual approach to determining the nature of the bonding of the sprayed coating to the substrate is destructive sectioning of the coated article and metallurgical inspection of the bondline region. This approach is normally applied to establish process parameters that achieve a good bonded coating, and then the same process parameters are used in the production coating operations. This approach has the drawback that, because the thermal-spray process is so versatile, it is difficult to perform the destructive testing over the entire range of possible types of coatings and configurations of substrate articles. Even if a process is established as acceptable with the destructive testing of test coupons, relatively minor variations in production parameters may lead to unacceptable bondline structures in the production articles. Another problem with the use of test coupons in this circumstance is that the test coupons may behave differently than the production parts. Post-coating operations such as heat treating and machining may introduce bondline defects to initially defect-free bondlines.
There is a need for an improved approach to the thermal-spray coating process and the evaluation of the bondline region of the coated article. The present invention fulfills this need, and further provides related advantages.